Commit message: cgroup: verify that source is a string

The following sequence can be used to trigger a UAF:

    int fscontext_fd = fsopen("cgroup");
    int fd_null = open("/dev/null, O_RDONLY);
    int fsconfig(fscontext_fd, FSCONFIG_SET_FD, "source", fd_null);
    close_range(3, ~0U, 0);

The cgroup v1 specific fs parser expects a string for the "source"
parameter.  However, it is perfectly legitimate to e.g.  specify a file
descriptor for the "source" parameter.  The fs parser doesn't know what
a filesystem allows there.  So it's a bug to assume that "source" is
always of type fs_value_is_string when it can reasonably also be
fs_value_is_file.

This assumption in the cgroup code causes a UAF because struct
fs_parameter uses a union for the actual value.  Access to that union is
guarded by the param->type member.  Since the cgroup paramter parser
didn't check param->type but unconditionally moved param->string into
fc->source a close on the fscontext_fd would trigger a UAF during
put_fs_context() which frees fc->source thereby freeing the file stashed
in param->file causing a UAF during a close of the fd_null.

Fix this by verifying that param->type is actually a string and report
an error if not.

In follow up patches I'll add a new generic helper that can be used here
and by other filesystems instead of this error-prone copy-pasta fix.
But fixing it in here first makes backporting a it to stable a lot
easier.

Fixes: 8d2451f4994f ("cgroup1: switch to option-by-option parsing")
Reported-by: syzbot+283ce5a46486d6acdbaf@syzkaller.appspotmail.com
Cc: Christoph Hellwig <hch@lst.de>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: <stable@kernel.org>
Cc: syzkaller-bugs <syzkaller-bugs@googlegroups.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
CVE: CVE-2021-4154
CWE: CWE-416
Vulnerability description: A use-after-free flaw was found in cgroup1_parse_param in kernel/cgroup/cgroup-v1.c in the Linux kernel's cgroup v1 parser. A local attacker with a user privilege could cause a privilege escalation by exploiting the fsconfig syscall parameter leading to a container breakout and a denial of service on the system.

Project: linux
File: cgroup-v1.c

int cgroup1_parse_param(struct fs_context *fc, struct fs_parameter *param)
{
	struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
	struct cgroup_subsys *ss;
	struct fs_parse_result result;
	int opt, i;

	opt = fs_parse(fc, cgroup1_fs_parameters, param, &result);
	if (opt == -ENOPARAM) {
		if (strcmp(param->key, "source") == 0) {
			if (fc->source)
				return invalf(fc, "Multiple sources not supported");
			fc->source = param->string;
			param->string = NULL;
			return 0;
		}
		for_each_subsys(ss, i) {
			if (strcmp(param->key, ss->legacy_name))
				continue;
			if (!cgroup_ssid_enabled(i) || cgroup1_ssid_disabled(i))
				return invalfc(fc, "Disabled controller '%s'",
					       param->key);
			ctx->subsys_mask |= (1 << i);
			return 0;
		}
		return invalfc(fc, "Unknown subsys name '%s'", param->key);
	}
	if (opt < 0)
		return opt;

	switch (opt) {
	case Opt_none:
		/* Explicitly have no subsystems */
		ctx->none = true;
		break;
	case Opt_all:
		ctx->all_ss = true;
		break;
	case Opt_noprefix:
		ctx->flags |= CGRP_ROOT_NOPREFIX;
		break;
	case Opt_clone_children:
		ctx->cpuset_clone_children = true;
		break;
	case Opt_cpuset_v2_mode:
		ctx->flags |= CGRP_ROOT_CPUSET_V2_MODE;
		break;
	case Opt_xattr:
		ctx->flags |= CGRP_ROOT_XATTR;
		break;
	case Opt_release_agent:
		/* Specifying two release agents is forbidden */
		if (ctx->release_agent)
			return invalfc(fc, "release_agent respecified");
		ctx->release_agent = param->string;
		param->string = NULL;
		break;
	case Opt_name:
		/* blocked by boot param? */
		if (cgroup_no_v1_named)
			return -ENOENT;
		/* Can't specify an empty name */
		if (!param->size)
			return invalfc(fc, "Empty name");
		if (param->size > MAX_CGROUP_ROOT_NAMELEN - 1)
			return invalfc(fc, "Name too long");
		/* Must match [\w.-]+ */
		for (i = 0; i < param->size; i++) {
			char c = param->string[i];
			if (isalnum(c))
				continue;
			if ((c == '.') || (c == '-') || (c == '_'))
				continue;
			return invalfc(fc, "Invalid name");
		}
		/* Specifying two names is forbidden */
		if (ctx->name)
			return invalfc(fc, "name respecified");
		ctx->name = param->string;
		param->string = NULL;
		break;
	}
	return 0;
}